Increasingly stringent federal regulations limit the permissible levels for emissions. As such, vehicle manufacturers have developed various methods to reduce emissions while improving vehicle performance and fuel economy. Catalytic converters are often used to reduce emission levels of regulated exhaust gases. The conversion efficiency of a catalytic converter may be monitored using a pre-catalyst oxygen sensor (HEGO sensor) positioned upstream from the catalytic converter and a post-catalyst oxygen sensor (catalyst monitor sensor or CMS) positioned downstream from the catalytic converter. One method known for indicating conversion efficiency of the catalyst is to calculate a ratio of CMS transitions or switches to HEGO transitions or switches. An increasing switch ratio is generally indicative of a degrading catalyst. When the switch ratio exceeds a threshold value, a malfunction indicator light (MIL) is illuminated so the vehicle operator will seek service. As such, it is important to properly set the threshold value to reduce unnecessary or premature service indications while eliminating the possibility of an unacceptable catalyst being undetected.
The existing method for determining the failure switch ratio threshold value is based on samples obtained from prototype development vehicles. A simulated aging process is applied to the catalytic converters to produce emission threshold converters, i.e., converters with an unacceptable degraded efficiency. The emissions threshold converters are installed on vehicles and tested under various operating conditions, such as varying ambient temperature, varying engine load, varying altitude, and the like, to determine representative switch ratios. This testing may span several months and requires numerous man-hours to complete. The collected switch ratios are then used to establish a statistically reliable failure switch ratio threshold, typically set to three standard deviations below the mean switch ratio of the emissions threshold converters.
Any changes made to the vehicle which may affect emissions levels require additional testing to verify or re-establish the failure threshold. In addition, while the simulated aging process provides a good estimate, it may not be truly representative of actual catalyst performance over the time represented by the simulation.